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        <h1 id="Mysql的基本架构"><a href="#Mysql的基本架构" class="headerlink" title="Mysql的基本架构"></a>Mysql的基本架构</h1><span id="more"></span>

<p><img src="https://cdn.jsdelivr.net/gh/DWJ-Squirtle/blogimage@main/image-20240417205313817.png" alt="image-20240417205313817"></p>
<h3 id="连接器"><a href="#连接器" class="headerlink" title="连接器"></a>连接器</h3><p>管理连接，权限验证</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">mysql -h$ip -P$port -u$user -p</span><br></pre></td></tr></table></figure>

<p>数据库里面，长连接是指连接成功后，如果客户端持续有请求，则一直使用同一个连接。短连接则是指每次执行完很少的几次查询就断开连接，下次查询再重新建立一个。<br>建立连接的过程通常是比较复杂的。所以尽量采用长连接，但是这样容易让Mysql内存涨的很快，有两个解决方案：</p>
<p>1、定期断开长连接。使用一段时间，或者程序里面判断执行过一个占用内存的大查询后，断开连接，之后要查询再重连</p>
<p>2、在每次执行一个比较大的操作后，通过执行mysql_reset_connection 来重新初始化连接资源。这个过程不需要重连和重新做权限验<br>证，但是会将连接恢复到刚刚创建完时的状态</p>
<h3 id="查询缓存"><a href="#查询缓存" class="headerlink" title="查询缓存"></a>查询缓存</h3><p>在mysql8.0之后就没有了</p>
<h3 id="分析器"><a href="#分析器" class="headerlink" title="分析器"></a>分析器</h3><p>分析器会先做词法分析，把输入的字符串中哪些是表名，哪些是列名识别出来，注意这个时候是会和真实的数据库有联系的，如果输入的字符串中的列名不存在，会直接在这一步就报错</p>
<p>之后是语法分析，判断输入的SQL语句是否符合Mysql语法</p>
<h3 id="优化器"><a href="#优化器" class="headerlink" title="优化器"></a>优化器</h3><p>优化器是在表里面有多个索引的时候，决定使用哪个索引；或者在一个语句有多表关联（join）的时候，决定各个表的连接顺序。比如你执行下面这样的语句，这个语句是执行两个表的 join：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">mysql&gt; select * from t1 join t2 using(ID) where t1.c=10 and t2.d=20;</span><br></pre></td></tr></table></figure>

<p>既可以先从表 t1 里面取出 c&#x3D;10 的记录的 ID 值，再根据 ID 值关联到表 t2，再判断 t2里面 d 的值是否等于 20。<br>也可以先从表 t2 里面取出 d&#x3D;20 的记录的 ID 值，再根据 ID 值关联到 t1，再判断 t1 里面 c 的值是否等于 10</p>
<p>这两种执行方法的逻辑结果是一样的，但是执行的效率会有不同，而优化器的作用就是决定选择使用哪一个方案。</p>
<h3 id="执行器"><a href="#执行器" class="headerlink" title="执行器"></a>执行器</h3><p>开始执行语句，首先判断有没有执行权限</p>
<p>有权限执行器就会根据表的引擎定义，去使用这个引擎提供的接口，一步步执行将结果返回客户端</p>

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